Acids And Bases - Water And Neutral Solutions - BBC

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In this guide

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  2. Video
  3. Test
  1. Acids and bases
  2. Hydrogen and hydroxide ions
  3. Water and neutral solutions
  4. Neutralisation
  5. Ionic equations and spectator ions
  6. Titrations

Water and neutral solutions

Water molecules can break down into hydrogen ions and hydroxide ions.

\(H_{2}O(l)\rightleftharpoons H^{+}(aq)+OH^{-}(aq)\)

This is a reversible reaction. A small proportion of water molecules break up to form hydrogen ions and hydroxide ions. Some of these hydrogen and hydroxide ions then react together again to form water molecules.

This is called an equilibrium and is present in water and all aqueous solutions.

In water and neutral solutions, the concentration of hydrogen ions is equal to the concentration of hydroxide ions.

All acidic solutions contain more hydrogen ions than hydroxide ions.

All alkaline solutions contain more hydroxide ions than hydrogen ions.

Three see-saws. The first represents an acidic solution. The H plus ions on one side vastly outnumber the O H minus ions on the other, so the see-saw is tipped towards the H plus side. The second see-saw describes a neutral solution. There are an equal number of H plus ions on one side as there are O H minus ions on the other. The see-saw is perfectly balanced. The third see-saw shows an alkaline solution. The H plus ions on one side are greatly outnumbered by the O H minus ions on the other side, so the see-saw tips towards the O H minus side.

Diluting acids and bases

Adding water to an acid or base will change its pH. Water is mostly water molecules so adding water to an acid or base reduces the concentration of ions in the solution.

When an acidic solution is diluted with water the concentration of H+ ions decreases and the pH of the solution increases towards 7.

To make the pH change by 1, a tenfold dilution is required (eg adding 9 cm3 of water to 1 cm3 acid). The acid is becoming less acidic.

Similarly, when an alkali is diluted with water the concentration of OH- ions decreases. This causes the pH of the alkali to fall towards 7, making the solution less alkaline as more water is added.

Next pageNeutralisationPrevious pageHydrogen and hydroxide ions

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